1. Field of the Invention
The present invention relates to methods for manufacturing high density memory devices based on phase change based memory materials, including chalcogenide based materials and other materials, and most particularly to methods for manufacturing a phase change memory element with a pillar-shaped bottom electrode.
2. Description of Related Art
Phase change based memory materials are widely used in nonvolatile random access memory cells. Such materials, such as chalcogenides and similar materials, can be caused to change phase between an amorphous state and a crystalline state by application of electrical current at levels suitable for implementation in integrated circuits. The generally amorphous state is characterized by higher resistivity than the generally crystalline state, which can be readily sensed to indicate data.
The change from the amorphous to the crystalline state is generally a low current operation. The change from crystalline to amorphous, referred to as reset herein, is generally a higher current operation, which includes a short high current density pulse to melt or breakdown the crystalline structure, after which the phase change material cools quickly, quenching the phase change process, allowing at least a portion of the phase change structure to stabilize in the amorphous state. It is desirable to minimize the magnitude of the reset current used to cause transition of phase change material from a crystalline state to amorphous state. The magnitude of the needed reset current can be reduced by reducing the size of the phase change material element in the cell and of the contact area between electrodes and the phase change material, so that higher current densities are achieved with small absolute current values through the phase change material element.
One direction of development has been toward forming small pores in an integrated circuit structure, and using small quantities of programmable resistive material to fill the small pores. Patents illustrating development toward small pores include: Ovshinsky, “Multibit Single Cell Memory Element Having Tapered Contact,” U.S. Pat. No. 5,687,112, issued 11 Nov. 1997; Zahorik et al., “Method of Making Chalogenide [sic] Memory Device,” U.S. Pat. No. 5,789,277, issued 4 Aug. 1998; Doan et al., “Controllable Ovonic Phase-Change Semiconductor Memory Device and Methods of Fabricating the Same,” U.S. Pat. No. 6,150,253, issued 21 Nov. 2000.
Another memory cell structure under development, referred to sometimes as a mushroom cell because of the shape of the active region on the bottom electrode in a typical structure, is based on the formation of a small electrode in contact with a larger portion of phase change material, and then a usually larger electrode in contact with an opposite surface of the phase change material. Current flow from the small contact to the larger contact is used for reading, setting and resetting the memory cell. The small electrode concentrates the current density at the contact point, so that an active region in the phase change material is confined to a small volume near the contact point. See, for example, Ahn et al., “Highly reliable 50 nm contact cell technology for 256 Mb PRAM,” VLSI Technology 2005 Digest of Technical Papers, pages 98-99, 14 Jun. 2005; Denison, International publication No. WO2004/055916 A2, “Phase Change Memory and Method Therefor,” Publication Date: 1 Jul. 2004; and Song et al., United States Patent Application Publication No. US 2005/0263829 A1, “Semiconductor Devices Having Phase Change Memory Cells, Electronic Systems Employing the Same and Methods of Fabricating the Same,” Publication Date: 1 Dec. 2005.
One prior art technique for making very small bottom electrodes, as described in the Ahn et al. publication, is referred to herein as a plug-in-via process, and includes forming a dielectric fill layer over circuitry for accessing the memory cell, etching vias in the dielectric fill layer to form an opening for making contact to the circuitry, and depositing electrode material into the via. The resulting structure is then planarized to expose the electrode material within the via. The phase change material is deposited and patterned in contact with the electrode. Although this technique is suitable for forming very small bottom electrode structures using plugs in vias, it has proved to suffer reliability and yield issues. For example, as described by Ahn et al., it has proven difficult to form reliable contact with the underlying access circuitry at the bottom of very small vias. This results in some cells in the array permanently disconnected from the access circuits.
Furthermore, Ahn et al. have related that it is difficult to ensure in the plug-in-via process that the areas of the exposed tops of the plug electrodes are uniform after the planarizing step across a large array of such cells. Since the area of the top surface of the bottom electrode affects current density in the phase change material, and is a critical dimension for phase change cells of this type, significant variations in operation of the cells in a single array result. This problem is exacerbated by the techniques used in attempts to successfully fill the vias, including depositing thin films and anisotropic etching of the thin films to form sidewall spacers with the vias. The nature of the process of forming sidewall spacers tends to round off the top edges of the vias, making the plug of electrode material within the via to have a top end with an expanding cross-section. Because it is difficult to planarize the resulting structure uniformly across an entire array within tolerances sufficient to avoid this expanding top end, the etch back will not remove the expanded top end completely for all of the cells and results in a significant variation in size of the exposed top surface of the bottom electrode plugs.
Yet another problem arises in the formation of plug in via electrodes, because of the difficulty of uniformly filling vias. In particular, due to the dynamics of thin film deposition within small holes, the plug that results may include a void where the top of the via closes off before it has been completely filled below. Planarizing the structure may open the void, and creating a hole in the top surface of the electrode plug. Such holes cause problems with successful formation of a layer of phase change material over the electrode.
It is desirable therefore to provide a method for manufacturing of memory cell structure with good control over the critical dimensions of the bottom electrode and over the electrical integrity of connections to the bottom electrode, which is reliable and manufacturable for high density integrated circuit memory devices.